1. Field of the Invention
This invention is directed to the science of Vertical Seismic Profiling (VSP) in a borehole that is being actively drilled into earth formations.
2. Discussion of the Prior Art
Vertical Seismic Profiling is a method for imaging subsurface formations in the general vicinity of a borehole. In one technique, a seismic receiver is lowered into the open hole to a plurality of discrete depth levels beneath the surface of the earth. At each depth level, a sound source or shot is triggered at or near the surface of the earth. The shot may be fired at one or more different locations that are horizontally offset from the well head. A recording means registers the travel time from each shot to the receiver at each assigned depth. The data are processed to obtain useful information about the subsurface formations.
In an alternate, somewhat more economical approach, the shot and receiver locations are reversed. One or more seismic receivers are located along the surface at selected horizontal offset distances from the wellhead and shots are triggered at a plurality of selected depth levels.
Both methods require that the drilling operation be shut down during the seismic research, a very expensive situation for both operator and investor.
In the process of drilling a well, a drill bit, sometimes consisting of toothed rollers, is mounted at the lower end of a rotating string of drill pipe. The rotating drill bit penetrates a formation by disintegrating the rock at the bottom of the hole, creating a considerable amount of vibration in the process. It would seem economical of rig time if the drill bit itself were used as a seismic energy source for VSP work, thereby avoiding rig down time.
U.S. Pat. No. 4,718,048 teaches a method for instantaneous acoustic logging within a well bore by making use of the energy produced by the drill bit. In essence, seismic receivers on the surface record the signals produced by the drill bit vibrations on a continuous basis. An accelerometer mounted at the top of the drill string records the drill bit vibrations as transmitted up the drill string. The accelerometer recording is cross-correlated with the signals recorded by the receivers to yield useful data about the subsurface formations.
There are certain problems in the above method. Some of those problems are addressed in U.S. Pat. No. 4,829,489, assigned to the assignee of this invention. In this patent there is taught a method for determining the drill string velocity. That quantity is needed in order to compensate the cross-correlations for progressive differences in the length of the drill string as the hole is deepened. U.S. Pat. No. 4,862,423 tells us how to reduce drill string multiples in the received field signals. Drill string multiples occur because vibrations are reflected repeatedly between discontinuities along the drill string as explained in the '423 patent. Drill string multiples are undesirable because they obscure the desired seismic events on a display.
In a recent paper that was published in Geophysics, v. 56, n. 5, May, 1991 by J. W. Rector III et al., the authors describe the field and analytical procedures useful in the employment of drill bit energy as a downhole seismic source. The reader is referred to that paper for its tutorial content.
All of the above methods consider the source of energy to be a discrete point source. I have discovered that the extensional waves propagating up the drill string due to the chattering of a drill bit at the lower end thereof, cause the entire drill string to act as a distributed seismic-energy source from which useful data can be obtained that are not available from discrete sources.